To establish the context of the present invention we need to examine the state of the prior art as it relates to the drive train of the suspension equipped off-road bicycle.
Careful engineering has nearly eliminated chain grow (the lengthening of the distance of the run of drive chain from the chain ring or rings to the rear drive cog or cogs) from the upper, drive or tension portion of the drive chain due to the activation of bicycle rear wheel suspension. The recent trend is to use what is colloquially called a “1-by” drive train configuration, where a single front chain ring is used in combination with an increasingly large plurality of rear cogs forming an increasingly wide-ranging cassette. This is displacing previous 3-by-9 and 2-by-10 drive trains with a more and more common 1-by-10 and 1-by-11 configuration. This transition has been aided by the proliferation of the chain guide and what can be generically described as the clutch derailer. It is even more recently being enhanced by the emergence of an alternating wide-narrow tooth profile being used on the chain ring to allow a tighter fit between chain and chain ring. This allows the tension portion of the chain to be more directly guided onto and supported by the chain ring even when the chain line is offset due to shifting the chain across the plurality of rear cogs. This wide-narrow tooth profile can be used on a chain ring in a 1-by drive train since there is no longer a need to allow for ease of derailment. The clutch derailer serves to offer unidirectional resistance to cage movement to reduce vibration-induced motion of the lower portion of the drive chain (the return portion that is not under drive tension). This is commonly known as “chain slap”. This additional resistance to slackening of the lower portion of the drive chain is often serving to reduce an inertial “whip effect”. This is caused by halted downward inertia of the bicycle, such as when landing from a drop, causing the chain to continue downwards. With a traditionally sprung rear derailer, the inertia of the chain pulls the derailer cage forward which allows the inertia to progress forward in a wave or whip-like fashion to derail the chain from the bottom of the chain ring in the absence of a lower chain guide system. While the technology described above serves to address many problems, there remains an engineering challenge.
In many suspension designs there is a lengthening effect along the lower portion of the drive chain on suspension activation. This has previously been addressed by the inherent ability of the rear detailer to accommodate variations in effective chain length. Recent technological efforts and developments indicate that there is growing concern for the lower portion of the chain. The prior art suspension designs that do not address lower portion chain grow place additional requirements for what is called “derailer capacity”; this is the ability to handle a difference in effective chain length as a drive train shifts through the gears. It is common practice to verify correct and adequate chain length while a suspension linkage is moved through its entire range of motion. Failure to accommodate any suspension induced chain grow can have costly destructive effects. Even if the extra capacity requirement of the suspension design itself is addressed by proper chain length and sufficient derailer capacity, chain grow places an additional force on both clutch type and traditionally sprung rear derailers.
The effect of suspension induced chain grow on the derailer is greater than the inertial effect of the chain moving downwards because of the ratio of mass involved. The inertia of the lower portion of the chain comes from the mass of approximately half of the total length of chain; the chain grow effect is based on the downward inertia applied by the combined mass of the rider and bicycle.
We can also choose to examine a more subtle effect of chain grow in terms of static and kinetic friction. The clutch derailer produces a static friction against cage rotation to reduce chain slap, once activated by a strong mechanical force such as shifting the chain to a larger cog or suspension induced chain grow, the clutch is momentarily in a state of kinetic friction. Since the kinetic friction is less than static friction, once in rotation, the clutch has less resistance to motion. If activated by the suspension, a clutch derailer has less effectiveness against chain slap or inertial whip effect.
In light of the discussion above there exists a need to address suspension induced lower portion chain grow to let each aspect of the recently advancing prior art individually and only handle the functions it best performs.